Meta‐analysis on the potential for increasing nitrogen losses from intensifying tropical agriculture

Abstract

Fertilized temperate croplands export large amounts of reactive nitrogen (N), which degrades water and air quality and contributes to climate change. Fertilizer use is poised to increase in the tropics, where widespread food insecurity persists and increased agricultural productivity will be needed, but much less is known about the potential consequences of increased tropical N fertilizer application. We conducted a meta‐analysis of tropical field studies of nitrate leaching, nitrous oxide emissions, nitric oxide emissions, and ammonia volatilization totaling more than 1,000 observations. We found that the relationship between N inputs and losses differed little between temperate and tropical croplands, although total nitric oxide losses were higher in the tropics. Among the potential drivers we studied, the N input rate controlled all N losses, but soil texture and water inputs also controlled hydrological N losses. Irrigated systems had significantly higher losses of ammonia, and pasture agroecosystems had higher nitric oxide losses. Tripling of fertilizer N inputs to tropical croplands from 50 to 150 kg N ha−1 year−1 would have substantial environmental implications and would lead to increases in nitrate leaching (+30%), nitrous oxide emissions (+30%), nitric oxide (+66%) emissions, and ammonia volatilization (+74%), bringing tropical agricultural nitrate, nitrous oxide, and ammonia losses in line with temperate losses and raising nitric oxide losses above them.

Nitric and nitrous oxide fluxes from intensifying crop agriculture in the seasonally dry tropical Amazon–Cerrado border region

“…However, doubling fertilizer levels from 80 to 160 kg N ha ‒1 yr ‒1 in soybean–maize double‐cropped systems increased NO fluxes by an order of magnitude, from 0.6 to 6.7 kg N ha ‒1 mo ‒1 , which would roughly scale to 1.1 and 7.4 kg N ha ‒1 yr ‒1 , much higher than most observations from a NO fluxes at a 160‐kg N ha ‒1 yr ‒1 fertilization rate that were typically <2 kg N ha ‒1 yr ‒1 at the same fertilization rate in a global meta‐analysis (Huddell et al., 2020). Nitric oxide fluxes of 7.4 kg N ha ‒1 yr ‒1 exceeded the expected value for NO fluxes at a 160‐kg N ha ‒1 yr ‒1 fertilization rate, but well within the range of observations from a global meta‐analysis on fertilized croplands (Huddell et al., 2020). In the high fertilization treatment, our hourly NO flux measurements were among the highest ever measured in agricultural systems, comparable with the record highs reported in Matson (1998) and Oikawa et al.…”

“…The relationships between N fertilizer use and fluxes of NO and N 2 O are fairly well established in temperate agroecosystems, but unintended N losses from fertilizer are poorly understood in tropical croplands. A recent meta‐analysis identified only 11 studies with NO measurements from fertilized agricultural systems in the tropics (Huddell et al., 2020). Although across the global tropics increased fertilizer use results in larger emissions of N 2 O and NO, these emissions are highly variable from site to site (Huddell et al., 2020).…”

“…A recent meta‐analysis identified only 11 studies with NO measurements from fertilized agricultural systems in the tropics (Huddell et al., 2020). Although across the global tropics increased fertilizer use results in larger emissions of N 2 O and NO, these emissions are highly variable from site to site (Huddell et al., 2020). Although N 2 O emissions from croplands in tropical and subtropical Brazil at current levels of N fertilization have been found to be smaller than responses to similar levels of fertilization of temperate croplands (Meurer et al., 2016), NO emissions may be higher (Huddell et al., 2020).…”

Disruption of the global nitrogen cycle: A grand challenge for the twenty-first century

“…The fate of the increase in N fertilizer to croplands in SSA and other tropical regions is not well studied and is a high priority research area for the next decade. Recently, a metaanalysis of the risk for increasing N losses from intensifying tropical agriculture, including in SSA, concluded that inputs from 50 to 150 kg N ha -1 year -1 would have substantial environmental consequences and lead to large increases in NO 3 leaching and gaseous nitrogen fluxes to the atmosphere from many, but not all tropical soils (Huddell et al 2020). Soil orders such as Oxisols and some Ultisols, which cover about 20% of SSA (Eswaran et al 1998), have net anion exchange capacity that decreases NO 3 leaching into the environment (Russo et al 2017).…”

Feeding Efficiency Gains Can Increase the Greenhouse Gas Mitigation Potential of the Tanzanian Dairy Sector

“…However, higher total energy content of diets can also increase methane production per animal [17] . Other risks include increasing CO 2 emissions from expanding cropland areas [18] and N 2 O emissions from intensi cation of feed crop production [19] . Changes in feeding practices can also lead to land sparing by substituting low yielding grass and forages with higher yielding feed crops, for which regional and global studies have suggested can reduce grassland requirements [20] and reduce deforestation [21] .As an estimated 96% of cattle in Tanzania are reared in extensive grazing systems [22] , we hypothesized that land sparing is a leading strategy for reducing dairy emissions.…”